1. Field of the Invention
The present invention relates to a piezoelectric member, a piezoelectric element, and a liquid discharge head and a liquid discharge apparatus utilizing the piezoelectric element.
2. Description of the Related Art
Recently, piezoelectric actuators are attracting attention, in the fields of mobile information equipment, chemistry and medical technology, as a novel motor replacing the electromagnetic motor, as it enables miniaturization and higher density in the motor structure. The piezoelectric actuator, in the driving thereof, does not generate electromagnetic noises and is not influenced by external noises. Also the piezoelectric actuator is attracting attention as a technology of realizing equipment of a submillimeter size as represented by micromachines, and a very small piezoelectric element is required as a driving source therefor.
In general, a piezoelectric element has a construction in which a pair of electrodes is connected to a piezoelectric member. The piezoelectric member is generally produced from a material having piezoelectric property such as a sintered member of a heat treated bulk material or a single crystal member, by forming into a small size of desired dimension and thickness by working technologies such as cutting and polishing. Also for producing a minute piezoelectric element, employed generally is a method of coating the piezoelectric member in a state of a green sheet for example by a printing method in a predetermined position on a substrate such as a metal or silicon and sintering such piezoelectric member thereby directly preparing a piezoelectric element. The member formed from such green sheet generally has a thickness of from tens to hundreds of micrometers, and, on and under the piezoelectric member, disposed are electrodes through which a voltage is applied.
Conventionally, a piezoelectric member to be used in a small piezoelectric element, as employed in a liquid discharge head, has also been produced from the aforementioned material by forming into a small form with working technologies such as cutting and polishing or from a piezoelectric member of the green sheet state. Examples of the apparatus utilizing such piezoelectric element include a liquid discharge head having a unimorph type piezoelectric element structure. The liquid discharge head is equipped with a pressure chamber communicating with an ink supply chamber and an ink discharge port communicating with the pressure chamber, with a vibrating plate, on which a piezoelectric element is adjoined or formed directly, equipped in the pressure chamber. In such construction, a predetermined voltage is applied to the piezoelectric element to cause an extension-contraction deformation therein thereby inducing a bending vibration and pressurizing the ink in the pressure chamber, thus discharging an ink liquid droplet from the ink discharge port.
Color ink jet printers utilizing the aforementioned function of the piezoelectric member are currently popular, but, also for such printers of piezoelectric type, desired are improvements in the printing ability, in particularly a higher resolution and a higher printing speed. For this reason, it has been tried to realize a higher resolution and a higher printing speed by miniaturizing the liquid discharge head to attain a multi-nozzle head structure. For the miniaturization of the liquid discharge head, a further miniaturization in the piezoelectric element for ink discharge is necessary. Also attempts are being actively made in applying the liquid discharge head in industrial applications such as directly drawing of a wiring. For such applications, being required is a liquid discharge head having more diversified characteristics with a miniaturization and a higher performance in the structure of the element structure for generating the discharge pressure.
Together with the recent advancement in the micromachine technologies, researches are being made for developing an ultra-small piezoelectric element of a higher precision by forming the piezoelectric member as a thin film and by utilizing the fine working technologies developed in the semiconductor field. The piezoelectric film, formed by a thin film forming method such as a sputtering method, a chemical vapor deposition method, a sol-gel method, a gas deposition method, or a pulsed laser deposition method, has a thickness of from about hundreds of nanometers to tens of micrometers in case of application to a piezoelectric actuator. Electrodes are connected to such piezoelectric film for applying a voltage.
On the other hand, with the compactification of the piezoelectric element, actively researched also are high-performance piezoelectric materials showing stronger piezoelectric characteristics. A piezoelectric material attracting attention recently is a composite oxide material having a perovskite type structure represented by a general formula ABO3. Such material, as represented by Pb(ZrxTi1-x)O3 (lead zirconate titanate: PZT), is excellent in dielectric property, pyroelectric property and piezoelectric property. Examples of the PZT material include those described in Non-patent Reference 1.
It is generally considered that a high piezoelectric property can be obtained by applying an electric field in a direction of spontaneous polarization of a piezoelectric member formed by a bulk single crystal, but, as a method for improving the piezoelectric property of the piezoelectric material, researches are being made on a domain control, called domain engineering. For example, a particularly excellent piezoelectric property is exhibited by relaxer type single crystal materials as represented for example by [Pb(Mg1/3Nb2/3)O3]1-x—(PbTiO3)x (lead magnesate niobate titanate: PMN-PT) and [Pb(Zn1/3Nb2/3)O3]1-x—(PbTiO3)x (lead zincate niobate titanate: PZN-PT). For example the Patent Reference 1 discloses a method of synthesizing PMN-PT by flux melting. It is reported that a bulk single crystal member was obtained by such synthesis and a material having a large strain amount exceeding 1% could be obtained. Also Non-patent Reference 2 reports that a domain control, called engineered domain structure, on PZN-PT could obtain a piezoelectric constant of 30 times or more (2500-2800 μC/N) of the piezoelectric constant d33 in the direction of spontaneous polarization. However, such bulk piezoelectric member has to be formed into a small size by the aforementioned technologies of cutting and polishing, and is difficult to apply to an ultra-small piezoelectric element of a higher precision.
It is therefore investigated to form such piezoelectric member as a film by a thin film forming method such as a sputtering method, a chemical vapor deposition method, a sol-gel method, a gas deposition method or a pulsed laser deposition method. However, even with the material of the high piezoelectric property as described above, the piezoelectric member (piezoelectric film) prepared by the thin film forming method has often not realized a high piezoelectric property as anticipated, with a significant difference.
It is also investigated to form a piezoelectric film of a relaxer type material by a thin film forming method such as a sputtering method, a chemical vapor deposition method, a sol-gel method, a gas deposition method or a pulsed laser deposition method. For example Non-patent Reference 3 reports that a PMN-PT thin film was formed by the PLD method.
Also as another method of domain control called domain engineering, investigated is an attempt to obtain a piezoelectric deviation larger than the polarization deviation inherent to the material itself, based on a rotation of a domain that is not horizontal to the electric field (for example a domain having a polarization in a substantially perpendicular direction) in the polarization direction. Such rotation of the polarization direction is generally called a 90° domain switching. For example, in a piezoelectric film of <100> orientation, there exists a phenomenon that a domain of [100] orientation is switched to a domain of [001] orientation by an electric field application in the film thickness direction. However, a very high energy is required for expressing such piezoelectric deviation and it is difficult to switch all the domains of [100] orientation to [001] orientation by the electric field application.
The Non-patent Reference 2 further reports that, as a direction for improving the piezoelectric property by the domain control, a reduction in the domain size is effective. The domain engineering enables to obtain a piezoelectric deviation larger than the polarization deviation inherent to the material itself, based on a phase change in the crystal resulting from an electric field application and on a rotation of the domain that is not horizontal to the electric field (for example domain having a polarization in a substantially perpendicular direction) in the polarization direction. However, a very high energy is required for expressing such piezoelectric deviation. For this reason, in the piezoelectric material utilizing the domain engineering, in order to reduce the energy of the piezoelectric deviation, it is important to have a domain structure that can induce a phase change of the crystal or a domain rotation in the piezoelectric material.
On the other hand, the present inventors have shown, in Patent Reference 2, that a twin crystal structure in the piezoelectric film improves the piezoelectric property and improves the adhesion of the piezoelectric film with the lower or upper electrode. It is estimated, as a reason therefor, the twin crystal structure present in the piezoelectric film may relax an internal stress, generated at the preparation of the material by various methods. It is therefore considered possible to obtain a piezoelectric element exhibiting a piezoelectric property close to that of the piezoelectric member obtained from a bulk material and exhibiting a satisfactory adhesion of the piezoelectric film with the lower or upper electrode.
Patent Reference 1: Japanese Patent 3397538
Patent Reference 2: Japanese Patent Application Laid-Open 2004-249729 (corresponding to U.S. Pat. No. 7,144,101)
Non-patent Reference 1: “Ceramic dielectric engineering” 4th edition, published Jun. 1, 1992, Gakken-sha, p. 333
Non-patent Reference 2: Ceramics, Vol. 40(8), 2005, p. 600
Non-patent Reference 3: Applications of Ferroelectrics, 2002, ISAF 2002, Proceedings of the 13th IEEE International Symposium, p. 133-136